As a method for driving a motor using a motor drive device, there is a known method for driving a motor, in which an AC voltage input from a main power supply is converted into a DC voltage using a converter and then this DC voltage is converted into an AC voltage using an inverter. Further, to smooth a DC voltage output from the converter, a DC link unit including a smoothing capacitor and others is provided. During an abnormality in the DC link unit, when AC power is applied to the converter from the main power supply, there is a possibility that the inverter may be damaged. When, for example, power of the main power supply is applied during short-circuit of the DC link unit, large current flows into the inverter, resulting in a risk that the motor drive device may be broken. Therefore, it is important to detect the presence or absence of an abnormality of the DC link unit before AC power from the main power supply is applied to the converter.
Heretofore, a motor drive device for detecting the presence or absence of an abnormality of a DC link unit has been disclosed (for example, refer to Japanese Unexamined Patent Application Publication No. 5-336759 (JP-A-5-336759)). FIG. 1 illustrates a configuration diagram of a conventional motor drive device. The conventional motor drive device includes a converter 1002 for rectifying an AC voltage to be input, smoothing means 1004 that is charged with an output voltage rectified by this converter 1002 to smooth the output voltage, current limiting means 1009a and 1009b for limiting a magnitude of charging current of this smoothing means 1004, an inverter 1005 using a semiconductor switching circuit for feeding an AC voltage to a load by PWM control of an output voltage of the smoothing means 1004, a voltage detection circuit 1010 for detecting a voltage of both ends of the smoothing means 1004, and determination means 1012 using a CPU for determining normality or abnormality based on whether an output voltage of this voltage detection circuit 1010 is larger or smaller than a predetermined voltage. In the conventional motor drive device, as illustrated in FIG. 1, a voltage has been applied from a main power supply 1001 to the smoothing means 1004 that is a DC link unit via the resistors 1009a and 1009b to detect the presence or absence of an abnormality based on a increasing speed of the voltage.
However, the conventional motor drive device is applied with a large voltage from the main power supply 1001 and therefore it has been necessary to allow a resistor to have large normal rated power and to be expensive and large in size.